The present invention relates to new ceramics and their preparation method. More particularly, it relates to novel ceramics made of flaky .beta.-SiC containing .beta.-SiC as the major component; a process for preparation of such materials; and their applications. The objects of the present invention are to provide new ceramics with improved properties in resistance to thermal shock, thermal fatigue and oxidation which are prepared from an organic silicon polymer compound (to be explained later in detail) as the starting material; and to manufacture novel ceramics and refractories with desirable physico-chemical properties by utilizing the above ceramics as the starting material.
Flaky .beta.-SiC of the present invention (or to be described as thin fragments or scales) is a hitherto unknown type of ceramics and is obtained by following method in which thin sheet is prepared with an organic silicon polymer compound containing the carbon and silicon atoms as the major skeletal components, the sheet is made infusible by conventional methods; the infusible sheet is cut into flakes and finally the flakes are heat-treated in the atmosphere of a non-oxidative gas. This material finds utility in the starting material for new sintered ceramic compact and refractories having a unidirectional or random laminary structure. The organic silicon polymer employed in the present invention is a high-molecular organic silicon compound containing the silicon and carbon atoms as the major skeletal components which has been synthesized by Prof. Yajima et al., the Research Institute for Iron, Steel and Other Metals, Tohoku University. It is well known that SiC fibers derived from such organic silicon polymers have also become well known world-wide by many papers and patents filed by Prof. Yajima et al.
The organic silicon polymer has opened a new field of research in materials and is utilized as the starting material for fibers as well as for binders, impregnants and coating compounds.
The present inventors have succeeded in preparation of a new sheet or flaky .beta.-SiC from the organic silicon polymer and one of the major objects of the present invention is to provide special thermo-resistant ceramics having excellent resistance to thermal shock, thermal fatigue and oxidation by utilizing such flaky .beta.-SiC.
Fundamentally speaking, the organic silicon polymers employable in the present invention have the following unit structures: ##STR1## wherein R.sub.1 is --CH.sub.3 ; and R.sub.2, R.sub.3 and R.sub.4 are one or more members selected from the group consisting of hydrogen, alkyl, aryl, (CH.sub.3).sub.2 CH--, (C.sub.6 H.sub.5).sub.2 SiH-- and (CH.sub.3).sub.3 Si--.
k, l, m and n show the numbers of repetition of the unit structures defined by ( ) and [ ], and usually vary in the following ranges: k=1-80; 1=15-350; m=1-80; n=15-350. The average molecular weight of the organic silicon polymers is in the range of 800-20000.
In unit structure III, M is a metallic or nonmetallic element such as Si, B, Ti, Fe, Al, Zr, Cr and the like, and may be contained in the starting material and/or is mixed in the starting material during the use of the catalyst employed for synthesis of the organic silicon polymer and is contained in the major skeletals. R.sub.5, R.sub.6, R.sub.7 and R.sub.8 are one or more members selected from the group consisting of hydrogen, alkyl, aryl, (CH.sub.3).sub.2 CH--, (C.sub.6 H.sub.5)2SiH-- and (CH.sub.3).sub.3 Si--, but any one or more of R.sub.5, R.sub.6, R.sub.7 and R.sub.8 may be absent, depending on the valence of M and the unit structure.
(v) Compounds that contain any one or more of unit structures (i)-(iv) as partial unit structures in their chain or three-dimentional construction; or the mixture of such compounds.
The average molecular weight of the organic silicon polymer compounds employed as the starting material of the present invention is in the range of 800-20000 and advantageously in the range of 1000-5000 and has a slightly larger range than the polymers for spinning. It is preferable to remove preliminarily organic silicon polymers having an average molecular weight above 20000 because of difficulty in fusion and sheet manufacture. It is also advisable to remove as much as possible organic silicon polymers having an average molecular weight below 800 and contaminating compounds having a low boiling point, because the presence of such compounds may cause some troubles in anti-fusion treatment, flaking and sintering, namely perforation, sticking and irregular thickness of the final products respectively.
The average molecular weight of the organic silicon polymer according to the present invention (M=.SIGMA.MiNi/.SIGMA.Ni) is determined in tetrahydrofuran at 20.degree. C. by osmometry using a vapor pressure osmometer.
The fusion and softening point of these polycarbosilicon compounds varies depending on the distribution curve of the molecular weights and usually is in the range of 100.degree. C. to 350.degree. C. When the organic silicon polymers in block, gel or powder state are heated directly in the atmosphere of a non-oxidative gas, they change to the liquid of low viscosity at a temperature of 100.degree. C. to 350.degree. C.
The organic silicon polymers molded into plate and thin membrane-like sheets silicon polymers is also easily liquefied, deformed or gelled upon heating in the atmosphere of a non-oxidative gas at a temperature of 100.degree. C. to 350.degree. C., giving rise to inseparable products. In the method according to the present invention, the organic silicon polymer molded in a thin sheet is preliminarily subjected to heat treatment at a temperature of 50.degree. C. to 400.degree. C. in the atmosphere of an oxidative gas such as the air, oxygen and ozone (anti-fusion treatment) so that the initial shape of the product may be maintained during the subsequent heat treatment in the atmosphere of a non-oxidative gas.
When the air is used as the oxidative gas which is relatively mild and easily controllable, the temperature is slowly raised to 70.degree. C. during a period of more than 20 minutes, and preferably of 40-100 minutes, and the anti-fusion treatment is carried out at a temperature of 70.degree. to 400.degree. C., and preferably of 120.degree. to 240.degree. C., for a period of 30 minutes to 5 hours, and preferably of 1 hour to 3 hours. This anti-fusion treatment gives a uniformly thick and least wavy sheet, which assures no troubles such as shrinking, deformation, irregular thickness and perforation of the final product in the subsequent heat treatment.
It should be remembered, however, that a thickness of a sheet above 100.mu. before anti-fusion treatment often leads to irregular thickness or wavy surface on the subsequent heat treatment. Thus the upper limit of thickness of a sheet should be 100.mu.. In addition, when a sheet is less than 10.mu. thick before anti-fusion treatment, the handling thereof becomes extremely difficult and the occurrence of laceration and perforation during the anti-fusion treatment is clearly confirmed under a microscope.
Therefore the present inventors have defined the average thickness of the thin sheet for production of flaky materials to between 10.mu. to 100.mu..
Before describing the heat treatment of an infusible sheet specifically, it seems useful to explain some general scientific observations on the change of the organic silicon polymer sheet which is provided with the anti-fusion treatment. When an infusible organic silicon polymer sheet is heated to a high temperature above its melting point in the atmosphere of a non-oxidative gas such as N.sub.2, H.sub.2, NH.sub.3, Ar and CO gas, R.sub.1 -R.sub.8 in the aforementioned unit structures of the organic silicon polymer compounds (a hydrogen atom, alkyl, aryl, (CH.sub.3).sub.2 CH--, (C.sub.6 H.sub.5)SiH-- and (CH.sub.3).sub.3 Si--) begin to escape as volatile breakdown products around 300.degree. C., while the skeletal carbon and silicon components become amorphous, and the formation of .beta.-SiC starts at a temperature around 800.degree. C.
At this stage, several to several hundreds molecules of .beta.-SiC are formed without regular crystal lattice from the amorphous material mainly composed of Si and C. In other words, several to several hundreds molecules of .beta.-SiC are present in the dispersed state in the carbon-rich amorphous material mainly composed of Si and C.
As the temperature rises above 1000.degree. C. and subsequently above 1200.degree. C., the production of .beta.-SiC from the amorphous phase rapidly increases and consequently the percentage of the amorphous phase decreases, while the carbon excess advances in the amorphous phase.
The amorphous phase of the Si-C system obtained after heat treatment to a temperature below 1000.degree. C. is still labile and unfavorable for subsequent processing. As the formation of .beta.-SiC is relatively abundant and the activity of the amorphous phase diminishes at a temperature above 1200.degree. C., it is possible to handle the sheet as the stable flaky material even in the presence of oxygen.
Upon heating to a temperature above 1500.degree. C. in the atmosphere of a non-oxidative gas, mainly .beta.-SiC and carbon are produced.
The thermal treatment at a temperature above 1800.degree. C. is not satisfying, because the flaky material becomes fragile and loses mechanical strength.
Based on these observations, the present inventors have determined the range of the heating temperature for synthesis of the flaky products between 1200.degree. C. to 1800.degree. C.
The above describes the fundamental findings on the production of flaky .beta.-SiC from the organic silicon polymer and its preparation method.
In the following, several processes are presented for preparation of .beta.-SiC.